Lutropin receptor (LHR) belongs to the superfamily of sever transmembrane domain G-coupled receptors. Its long extracellular aminoterminal polypeptide of 341-368 amino acid residues contains the high affinity hormone binding sequences. Since it has been possible to prepare fair amounts of the extracellular aminoterminal polypeptide from E. coli and CHO cells, it has now become feasible to undertake its detailed physicochemical, immunological and biological characterization. The ultimate goal of these studies is to define as precisely as possible the ligand binding site of the receptor with the objective of developing antagonists of human choriogonadotropin (hCG). The extracellular domain of rat (LHR) (1-294 and 1-341 residues) will be prepared in milligram quantities from E. coli, CHO and SF-9 insect cells. After initial characterization of the recombinant aminoterminal peptides of rat LHR by amino acid composition, peptide mapping and amino and carboxyterminal sequencing, (1) studies on the determination of the ligand binding site will be undertaken. Several approaches such as the use of deletion and truncation mutants by site directed mutagenesis, use of proteolytic fragments of the extracellular domain polypeptides and synthetic peptides will be made to map the high affinity hormone binding site of LHR. (2) The LHR will be further characterized by studying some of the posttranslational modifications including the positions of the disulfide bonds and the detailed structures of the carbohydrate unites in the extracellular domains from CHO and SF-9 insect cells. (3) For three dimensional structural analysis and attempt will be made to crystallized the aminoterminal 1-294 and 1-341 residue polypeptides as well as its selenomethionly analogs. Insertion of selenium will obviate the problem of isomorphous replacement by heavy metal ion in x-ray diffraction studies. (4) Polyclonal and monoclonal antibodies which inhibit the binding of hCG to the receptor. (5) Finally, the in vitro and in vivo biological characterization of the extracellular domain of rat LHR by studying the inhibition of hCG action in hCG induced increase in rat uterine weight, ovulation and progesterone synthesis will be carried out. These studies will help in the evaluation of the potential of the extracellular polypeptides of LHR as antagonists of hCG for fertility regulation. Initial studies on rat LHR will be extended to human LHR.